1. Field of the Invention
The present invention relates to a laser-diode-pumped solid-state laser having a solid-state laser rod that is pumped by a semiconductor laser in the form of a laser diode, and more particularly to a laser-diode-pumped solid-state laser having a wavelength conversion capability.
2. Description of the Prior Art
Japanese Unexamined Patent Publication No. 62(1987)-189783, for example, discloses a laser-diode-pumped solid-state laser having a solid-state laser rod doped with a rare-earth element such as neodymium (Nd), the solid-state laser rod being pumped by a semiconductor laser.
According to one conventional design involving such a laser-diode-pumped solid-state laser, the resonator houses a bulk single crystal of a nonlinear optical material for converting the wave of a solid-laser-oscillated beam into a second harmonic thereby to produce a laser beam of a shorter wavelength, as disclosed in U.S. patent application Ser. No. 406,018, now U.S. Pat. No. 5,124,999 for example. Another proposal, discussed in Applied Physics Letter, Vol. 52, No. 2, 11 Jan. 1988, also employs a bulk single crystal of a nonlinear optical material disposed in a resonator, for converting a solid-laser-oscillated beam and a pumping laser beam into a laser beam having a frequency that is equal to the sum of the frequencies of the solid-laser-oscillated beam and the pumping laser beam.
SPIE, Vol. 1104, p. 100, March 1989, shows a known solid-state laser rod of Nd:COANP, Nd:PNP, or the like that is doped with a rare-earth element such as neodymium and has a wavelength conversion capability. Another known solid-state laser rod disclosed in the same journal, p. 132 is made of Nd:LiNbO.sub.3, NYAB(ND.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4) (x=0.04.about.0.08). These materials are referred to as a self-frequency-doubling crystal.
There is known a laser-diode-pumped solid-state laser that employs a NYAB crystal and produces a second harmonic of a laser beam oscillated by laser-diode pumping, as disclosed in SPIE, Vol. 1104, p. 100, March 1989 and Laser Research, Vol. 17, No. 12, p. 48, 1989.
In the known laser-diode-pumped solid-state laser, the temperature of the semiconductor laser is controlled with an accuracy of .+-.0.1.degree. C. to equalize the wavelength of the laser beam emitted by the semiconductor laser to the wavelength of the laser beam absorbed by the solid-state laser rod for laser output power stabilization.
It has also been proposed in U.S. Pat. No. 4,884,277 to regulate the temperature of a resonator with an accuracy of .+-.0.1.degree. C. when the laser beam emitted by a laser-diode-pumped solid-state laser is converted into a wavelength-converted wave by two waveform converters, for stabilizing the output power level of the wavelength-converted wave and reducing the noise of the wavelength-converted wave.
However, the conventional laser-diode-pumped solid-state laser has been disadvantageous in that the temperature of the resonator cannot sufficiently be regulated, resulting in output power variations due to temperature-dependent changes in the resonator length, temporary output interruptions owing to mode hopping, and noise in the wavelength-converted wave. These problems are especially conspicuous when the ambient temperature is changed, for example, from 20.degree. C. to 40.degree. C.